1. Field of the Disclosure
This disclosure generally relates to an imaging device, more particularly, to a digital imaging device with an enhanced dynamic range and an operating method thereof.
2. Description of the Related Art
An image sensor usually includes a sensor array which senses light energy and outputs an analog signal, e.g., a current signal, a voltage signal or charges. The analog signal is then processed by an analog preprocessing and a digital post-processing to be configured as frame data.
When a photoelectric element, e.g., photodiode, outputs a photocurrent not under an absolute zero temperature, the photocurrent usually accompanies with dark current, which affects a real signal of the light energy sensed by the photoelectric element. Therefore, conventionally a so-called black level calibration (BLC) is used to eliminate the dark current signal. The black level calibration is generally implemented by disposing the black pixel in the sensor array to generate dark current to be used as a calibration value. As the name implies, the dark pixel is shielded from receiving external light, and generally the black pixel is formed by covering a shielding layer over a normal pixel.
Referring to FIG. 1, it is a schematic diagram of a conventional image sensor 9, which includes a sensor array 91, a correlated double sampling circuit (CDS) 93, an amplifier 95, an analog to digital converter (ADC) 97, a black level average circuit 98 and a subtractor 99.
The sensor array 91 includes a plurality of black pixels 91b and a plurality of normal pixels 91n. The correlated double sampling circuit 93 temporarily stores analog pixel data sequentially outputted by pixels in each row of the sensor array 91. The amplifier 95 amplifies the analog pixel data and transmits the amplified data to the analog to digital converter 97 (for example, 11 bits ADC) for digitization, as shown in FIG. 2, wherein digital pixel data of the normal pixels 91n is within a digital range of 1024-2047, and the digital pixel data of the black pixels 91b is near the digital value of 1023. The black level average circuit 98 calculates an average value of the digital pixel data of the black pixels 91b as a black level calibration value (BLC). The subtractor 99 subtracts the black level calibration value BLC from the digital pixel data of the normal pixels 91n to allow the digital pixel data of the normal pixel 91n to be within a valid dynamic range of 0-1023.
However, as shown in FIG. 3, the analog pixel data outputted by the normal pixels 91n can be affected by the pixel itself, the correlated double sampling circuit 93 and the amplifier 95 to have an offset. The offset leads the analog pixel data inputted into the analog to digital converter 97 to be out of the dynamic range (as circled by dash lines) of the analog to digital converter 97, and lowers the valid dynamic range after the black level calibration.